The present invention relates to an image display apparatus, such as, liquid crystal display apparatuses (LCDs), plasma display panel apparatuses (PDPs), digital light processing display apparatuses (DLPs), field emission display apparatuses (FEDs), and electroluminescent display apparatuses (ELs), particularly, to an image display apparatus for displaying images of digital input image signals through division of one frame to a plurality of subframes.
A recent panel-type image display apparatus, such as, LCD, PDP, DLP, FED, and EL employs a drive system for digital input image signals, which is quite different from known image display apparatuses with a cathode-ray tube (CRT). The panel-type image display apparatus displays images through division of one frame of an image signal into a plurality of subframes for representing a plurality of gradation levels (refer to Japanese Patent No. 349864, for example). Moreover, the panel-type image display apparatus requires reverse-gamma correction of an input image signal which has already been applied reverse-gamma characteristics, through a built-in reverse-gamma correction circuit, for an output signal (luminescence intensity) the characteristics of which linearly varies against the input signal.
The panel-type image display apparatus provides step-by-step gradation representation due to image display through digital driving, with application of reverse-gamma characteristics to an input image signal, resulting in difficulty in gaining correct gradation characteristics, particularly, for the image signal at lower gradation levels. It is thus customary to install a quasi-intermediate gradation signal generating circuit using dither or error diffusion to achieve quasi-intermediate gradation representation between adjacent gradation levels, as disclosed in the Japanese Patent.
Such known quasi-intermediate gradation through a quasi-intermediate gradation signal generating circuit has, however, difficulty in achieving further multi-gradation and hence cannot meet increased demand of multi-gradation in image display apparatuses. Enhancement of representable gradation levels could be achieved by increasing the number of subframes within one frame, which inevitably requires a higher operating frequency for an image display apparatus. A higher operating frequency necessitates modification to basic design of an image display apparatus. It is however unacceptable to raise an operating frequency due to the fact that there is limitation on increase in operating frequency for the integrated circuitry to drive an image display apparatus, and a higher operating frequency causes excess heat. Especially, PDPs suffer a lowered intensity when the number of subframes is increased, thus multi-gradation through increase in subframe numbers is not a feasible way.